Boost C++ Libraries: Ticket #3225: boost::fusion sequence throws compiler error when used with boost::mpl::is_sequence

attachment set

Fri, 26 Jun 2009 21:04:53 GMT

attachment → compiler_output.txt

compiler output

Fri, 26 Jun 2009 21:34:32 GMT

I'm also not having any luck using boost::fusion::vector with boost::mpl::fold and boost::mpl::find_if, so as a workaround for the moment I'll write an explicit conversion metafunction to boost::mpl::vector...

Joel de Guzman — Sat, 27 Jun 2009 00:49:38 GMT

You should probably ask the MPL authors. The thing is: boost::mpl::is_sequence is not a requirement to be a valid MPL sequence:

http://www.boost.org/doc/libs/1_39_0/libs/mpl/doc/refmanual/forward-sequence.html

as for fold and find_if, you need to provide a test case, add another trac ticket for those.

I would suggest though to work initially with mpl sequences and only convert to a fusion sequence (e.g using fusion::as_vector) only when necessary. Fusion sequences have more overhead compared with mpl sequences because of its runtime aspects.

Sat, 27 Jun 2009 03:40:33 GMT

Thanks for the quick reply!

My intent was to provide a compile-time check that the types within the boost::fusion sequence satisfied certain properties. Any boost::mpl-compatible code could thus be used for such checks, but some of the code I've written using boost::mpl has asserts that the sequences satisfy boost::mpl::is_sequence. Otherwise, I would indeed construct the type sequence through boost::mpl and at the end convert it to a boost::fusion sequence, as suggested in the documentation.

It's not just that a boost::fusion sequence fails to satisfy boost::mpl::is_sequence, but that attempting to apply a boost::fusion sequence to boost::mpl::is_sequence results in a compiler error, which is not the case for other non-boost::mpl-sequence types (I think). The compiler error looks somewhat similar to what happens when trying to apply boost::mpl::fold to a fusion sequence, so I'll post a trac ticket for that code example and see what you think before forwarding these problems onto boost::mpl.

Joel de Guzman — Sat, 27 Jun 2009 03:49:57 GMT

If there is code that relies on boost::mpl::is_sequence within MPL or outside, then, I'd say it is a bug. The requirements for a fully conforming MPL sequence is clearly stated in the reference (http://tinyurl.com/qy376b), and mpl::is_sequence is not one of the requirements.

I think this needs to be discussed in the Boost list. I'd love to be corrected, if I'm wrong.

status changed; resolution set

Joel de Guzman — Tue, 11 Nov 2014 23:40:09 GMT

status new → closed
resolution → fixed